System and method for forming a thin-film phosphor layer for phosphor-converted light emitting devices

ABSTRACT

A thin-film phosphor layer can be formed by an improved deposition method involving: (1) forming a phosphor powder layer that is substantially uniformly-deposited on a substrate surface; and (2) forming a polymer binder layer to fill gaps among loosely packed phosphor particles, thereby forming a substantially continuous layer of thin film.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application Ser.No. 61/114,198, filed on Nov. 13, 2008, the disclosure of which isincorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The invention relates generally to light emitting devices and moreparticularly to a thin-film phosphor deposition process for forming athin-film phosphor layer adjacent to semiconductor light emittingdevices.

BACKGROUND

Solid-State Lighting through Light Emitting Diodes (SSL-LEDs) involvesthe use of solid-state, inorganic semiconductor light emitting diodes toproduce white light for illumination. Like inorganic semiconductortransistors, which displaced vacuum tubes for computation, SSL-LED is adisruptive technology that has the potential to displace vacuum or gastubes used in traditional incandescent or fluorescent lighting.Advantages of SSL-LEDs over conventional light sources include: (1)higher efficiency and associated energy savings; (2) better colorrendering; (3) small form factor; (4) ruggedness; (5) longer operationallifetime and low maintenance; (6) environmentally friendly; and (7) lowfabrication costs.

Conventional LEDs typically generate monochromatic light with a narrowemission spectrum, and thus typically lack a broad emission spectrum toprovide white light for illumination. In order to generate white lightfrom an LED, a narrowband emission resulting from radiativerecombination in the LED is transformed into broadband white lightspectrum. Such broadband white light spectrum can be generated by threegeneral approaches. A first approach is a wavelength-conversion approachby using an ultraviolet (“UV”) LED to excite multi-color phosphors thatemit visible light at down-converted wavelengths. A second approach is acolor-mixing approach by combining multiple LEDs, each of whichgenerates light of a different color. A third approach is a hybridbetween the two approaches described above. The current generation ofcommercially available white LEDs is primarily based on this hybridapproach. In particular, primary light emitted from a blue InGaN-basedLED is mixed with a down-converted secondary light emitted from apale-yellow YAG:Ce³⁺-based inorganic phosphor. The combination ofpartially transmitted blue and re-emitted yellow light gives theappearance of cool (green-blue) white light. Thus, phosphor coatingtechnology is involved for white LEDs using either thewavelength-conversion approach or the hybrid approach.

Current approaches for phosphor coating are described next. A firstapproach, as depicted in FIG. 1A, is a slurry method involving the useof phosphor grains or particles 1 blended in a liquid polymer system,such as polypropylene, polycarbonate, epoxy resin, or silicone resin.The mixed phosphor slurry is dispensed on or surrounding an LED chip 2,and then the liquid polymer system is dried or cured. The LED chip 2along with the phosphor slurry can be disposed in a reflector cup 3, asdepicted in FIG. 1A. While the slurry method is a convenient phosphordispensing method, a resulting color uniformity of LEDs manufacturedwith this slurry method is typically unsatisfactory, and colored ringscan be observed from different viewing angles. These deficiencies arethe result of: (1) variations in the thickness of a phosphor-containingmaterial surrounding an LED chip can lead to various lengths of opticalpaths before an emitted light escapes a package; and (2) non-uniformphosphor distribution within the phosphor-containing material (becauseof gravity and buoyancy effects) tends to move larger phosphor particlesdownward during a liquid polymer curing process. Moreover, due tovariations in the quantity of phosphor powders dispensed surrounding theLED chip, a white color coordinate tends to vary from device to device.These color variations, in turn, result in a complicated white LED colorsorting process, the so-called color binning, which attempts to managethe color variations by sorting each device according to its white colorcoordinate.

To measure the uniformity of emitted light, the variation in aCorrelated Color Temperature (“CCT”) can be used. A color temperature ofa light emitting device can be determined by comparing its hue with atheoretical, heated blackbody radiator. A temperature, expressed interms of degrees Kelvin, at which the heated blackbody radiator matchesthe hue of the light emitting device is that device's color temperature.An incandescent light source can be close to being a blackbody radiator,but many other light emitting devices do not emit radiation in the formof a blackbody curve and are, therefore, assigned a CCT. A CCT of alight emitting device is a color temperature of a blackbody radiatorthat most closely matches the device's perceived color. The higher theKelvin rating, the “cooler” or more blue the light. The lower the Kelvinrating, the “warmer” or more yellow the light. By measuring the CCT atdifferent light emission angles and comparing this variation amongdifferent light emitting devices, the uniformity of the light producedcan be quantified. A blue LED chip dispensed with a yellow phosphor bythe slurry method can have a typical CCT that varies from about 5,800 Kto about 7,200 K across a range of 1,400 K for light emission angles at±70° from a center light-emitting axis of the LED. Because of thepresence of colored rings, the CCT is typically higher at or near thecenter axis than in the periphery, where the emitted light tends to bemore yellow.

A second phosphor coating method is an Electrophoretic Deposition(“END”) method for the manufacture of phosphor-converted white LEDs, asdepicted in FIG. 1B. In the case of END, a phosphor is electricallycharged by adding a proper amount of an electrolyte in a liquid solventto form a liquid suspension, and is biased by an electrical field. Then,surface charged phosphor particles are moved to an electrode ofcounter-polarity and coated on the electrode. EPD of the phosphorparticles creates a phosphor layer 4 of relatively uniform thicknessthat can produce white light of greater uniformity and reduced instancesof colored rings. While achieving greater color uniformity, the EPDmethod is generally lacking in its ability to deposit phosphors directlyover an electrically nonconductive surface. In commercial production, aphosphor layer is typically coated directly over a LED chip 5, accordingto the so-called proximate phosphor configuration. This configurationtends to be inefficient in terms of light scattering, since theproximate phosphor layer can direct about 60% of total white lightemission back towards the LED chip 5, where high loss can occur. Anotherdrawback of the EPD method is that certain phosphors are susceptible todegradation by the solvent, thereby limiting the general applicabilityof the END method.

More recently and as depicted in FIG. 2, another approach involvesforming a luminescent ceramic plate 6 by heating phosphor particles athigh pressure until surfaces of the phosphor particles begin to softenand melt. The partially melted particles can stick together to form theceramic plate 6 including a rigid agglomerate of the particles. Theluminescent ceramic plate 6 is disposed in a path of light emitted by anLED chip 7, which is disposed over a set of electrodes 8. Whileproviding benefits in terms of robustness, reduced sensitivity totemperature, and reduced color variations from chip to chip, a resultingpackage efficiency can be unsatisfactory due to the proximate phosphorconfiguration.

A scattering efficiency (also sometimes referred to as a packageefficiency) is typically between 40% to 60% for commercially availablewhite LEDs, with efficiency losses due to light absorption by internalpackage components such as an LED chip, a lead frame, or sub-mount. FIG.3 depicts an example of a phosphor-converted white LED with yellowphosphor 31 powered by a blue LED chip 32, where a primary blue light 34undergoing color mixing with a secondary light 35 of yellow color togenerate a white color. A main source of light loss results fromabsorption of light by the LED chip 32. Because the LED chip 32 istypically formed of high-refractive index materials, photons tend to betrapped within the LED chip 32 due to Total Internal Reflection (“TIR”)once the photons strike and enter the LED chip 32. Another potentialsource of light loss results from imperfections in a mirror reflector 33in the LED package.

Several scenarios depicted in FIG. 3 can direct light to the highlyabsorbent LED chip 32. First, a primary light 36 emitted by the LED chip32 can be reflected back to the chip 32 by the phosphor powders 31 or bythe mirror reflector 33. Second, down-converted secondary light 37emitted by the phosphor powders 31 can scatter backward towards the LEDchip 32. Third, both primary light and secondary light 38 can bereflected back towards the chip 32 due to TIR at an air-LED packageinterface. To improve the probability of light escaping from thepackage, a hemispheric lens 39 can be used to reduce instances of TIR atthe air-package interface. To reduce instances of backward scatteredlight striking the LED chip 32, the phosphor powders 31 desirably shouldnot be placed directly over the chip surface, but rather should beplaced at a certain distance from the LED chip 32. Furthermore, athinner phosphor layer would reduce instances of backward scattering ofsecondary light by the phosphor powders 31.

It is against this background that a need arose to develop the thin-filmphosphor deposition process and related devices and systems describedherein.

SUMMARY

Certain embodiments of the invention relate to the formation of athin-film phosphor layer of substantially uniform thickness that can beconformally disposed in an optical path of an LED, thereby producingsubstantially uniform white light with little or no colored rings. Thisthin-film phosphor layer can be prepared by an improved depositionmethod involving: (1) forming a phosphor powder layer that issubstantially uniformly deposited on a substrate surface; and (2)forming a polymer binder layer to fill gaps among loosely packedphosphor particles, thereby forming a substantially continuous,thin-film phosphor layer. Phosphor conversion efficiency of thethin-film phosphor layer can be significantly improved because a thinnerlayer of a precisely controlled quantity of phosphor powders can bedisposed in an optical path, thereby reducing light scattering losses.Also, color homogeneity of the thin-film phosphor layer can besignificantly improved due to substantially uniform deposition ofphosphor particles. One method of forming an uniform, thin-film phosphorlayer is to introduce electrostatic charges among phosphor particlesduring deposition of the phosphor particles. The electrostatic chargesamong the phosphor particles can self-balance and adjust theirdistribution, thereby promoting a substantially uniform distribution ofthe phosphor particles. Another method of forming an uniform, thin-filmphosphor layer is through a phosphor dispensing mechanism, such as ashowerhead mechanism in a deposition chamber, or through a rotationalsubstrate holding mechanism, such as a turn table that holds asubstrate. In addition to improved efficiency and color homogeneity,temperature stability of the thin-film phosphor layer can besignificantly improved because the polymer binder layer can be thermallystable up to at least about 300° C. or more.

Advantageously, white color consistency can be maintained in a tightcolor coordinate by a coating process with precisely controlledquantities of deposited phosphor particles through a phosphor powderdelivery mechanism. White color rendering can be precisely tuned with alayer-by-layer sequential deposition of multi-color phosphors, such asdeposition of a red phosphor layer, deposition of a green phosphorlayer, and then deposition of a blue phosphor layer. The ratio ofmulti-color phosphors can be precisely controlled in a resultingcomposite multi-color phosphor film stack. Thus, the color coordinateand CCT of a white LED fabricated by the phosphor thin-film process canbe precisely controlled. This, in turn, can significantly simplify (oreven avoid) a binning process.

According to some embodiments of the invention, a consistent white colorcoordinate can be achieved from lightly varied blue LED chips by tuningthe dosage of a multi-color phosphor film stack. This color compensationmethod can compensate for color variations of the blue LED chips usingdifferent compositions or amounts of phosphor contents. In such manner,white LED yield can be significantly increased for color sensitiveapplications, such as display backlighting using white LEDs.

According to one embodiment of the invention, a thin-film phosphorcoating method is a batch phosphor coating process. Multiple LED chipscan be deposited with thin-film phosphor in one coating operation.According to another embodiment of the invention, multiple LED lensescan be deposited with thin-film phosphor in one coating operation.Similar to semiconductor chip manufacturing, a manufacturing cost perLED chip can be significantly reduced, and a manufacturing throughputcan be significantly increased by a batch process.

As contrasted to EPD, deposition of a thin-film phosphor layer can beused to form conformal thin-film phosphor layers directly over anelectrically nonconductive surface. The conformal thin-film phosphoralso can be deposited on a non-flat surface, such as a convex or concavesurface of an LED lens.

Some embodiments of the invention relate to a system for depositing aconformal thin-film phosphor layer on a substrate. The system caninclude a deposition chamber, a phosphor powder delivery subsystem, anda polymer precursor delivery subsystem configured to deliver a gas phasemonomer to the deposition chamber. The system also can include a memoryand a processor in electrical communication with the deposition chamber,the phosphor powder delivery subsystem, and the polymer precursordelivery subsystem, wherein the memory can include code or instructionsstored therein and executable by the processor to form a thin-filmphosphor layer.

One specific embodiment of the invention relates to a method of forminga thin-film phosphor layer for use in a light emitting device. Themethod includes: (1) transporting, using a carrier gas, a phosphorpowder from a source of the phosphor powder to a deposition chamber; and(2) depositing the phosphor powder adjacent to a substrate within thedeposition chamber so as to substantially uniformly distribute thephosphor powder adjacent to a surface of the substrate.

Another specific embodiment of the invention also relates to a method offorming a thin-film phosphor layer for use in a light emitting device.The method includes: (1) forming a first phosphor powder layer adjacentto a substrate, the first phosphor powder layer including first phosphorparticles that are distributed adjacent to a surface of the substrate;and (2) forming, via vapor deposition, a first polymer layer adjacent tothe first phosphor powder layer, the first polymer layer serving as abinder for the first phosphor particles.

Another specific embodiment of the invention relates to a system to forma thin-film phosphor layer on a substrate. The system includes: (1) adeposition subsystem defining an enclosure to accommodate the substrate;(2) a phosphor powder delivery subsystem configured to deliver, using acarrier gas, a phosphor powder form a source of the phosphor powder tothe deposition subsystem; (3) a polymer precursor delivery subsystemconfigured to deliver polymer precursors in a vapor phase to thedeposition subsystem; and (4) a control subsystem connected to thedeposition subsystem, the phosphor powder delivery subsystem, and thepolymer precursor delivery subsystem, wherein the control subsystem isconfigured to control the phosphor powder delivery subsystem to deliverthe phosphor powder to the deposition subsystem for a first timeinterval to form a phosphor powder layer adjacent to the substrate, andthe control subsystem is configured to control the polymer precursordelivery subsystem to deliver the polymer precursors to the depositionsubsystem for a second time interval to form a polymer layer adjacent tothe phosphor powder layer.

Another specific embodiment of the invention relates to a light emittingdevice, which includes: (1) a substrate; (2) a phosphor powder layersubstantially uniformly distributed adjacent to the substrate; and (3) aparylene-based polymer layer conformally deposited adjacent to thephosphor powder layer as a binder material to form a substantiallycontinuous thin film, wherein the phosphor powder layer includes asingle-color phosphor.

A further specific embodiment of the invention relates to a lightemitting device, which includes: (1) a substrate; (2) multiple phosphorpowder layers substantially uniformly distributed adjacent to thesubstrate, the phosphor powder layers being configured to emit light ofdifferent colors; and (3) multiple parylene-based polymer layers, eachof the parylene-based polymer layers being conformally depositedadjacent to a respective one of the phosphor powder layers, each of theparylene-based polymer layers being a binder material for a respectiveone of the phosphor powder layers to fill voids among constituentphosphor particles.

Other aspects and embodiments of the invention are also contemplated.The foregoing summary and the following detailed description are notmeant to restrict the invention to any particular embodiment but aremerely meant to describe some embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the nature and objects of some embodimentsof the invention, reference should be made to the following detaileddescription taken in conjunction with the accompanying drawings.

FIG. 1A depicts a proximate phosphor-in-cup configuration of aconventional white LED formed using a slurry method.

FIG. 1B depicts a proximate phosphor configuration of a conventionalwhite LED formed using EPD.

FIG. 2 depicts a proximate phosphor configuration of a conventionalwhite LED formed by lamination with a luminescent ceramic plate.

FIG. 3 depicts sources of light losses resulting from light scatteringby phosphor powders, TIR at material interfaces, and light absorption ata surface of an LED.

FIG. 4 is a flow diagram depicting a method of forming a conformal,thin-film phosphor layer, according to an embodiment of the invention.

FIG. 5 is a flow diagram depicting a method of depositing asubstantially uniformly distributed phosphor layer, according to anembodiment of the invention.

FIG. 6A through FIG. 6D depict examples of a family of polymers that canbe used as binder materials to form thin-film phosphor layers, accordingto an embodiment of the invention.

FIG. 7A depicts a single-color thin-film phosphor layer formed using aconformal, thin-film phosphor deposition process, according to anembodiment of the invention.

FIG. 7B depicts a multi-color thin-film phosphor composite layer formedusing a conformal thin-film phosphor deposition process, according to anembodiment of the invention.

FIG. 8 depicts a system for depositing a conformal, thin-film phosphorlayer on a substrate, according to an embodiment of the invention.

DETAILED DESCRIPTION Definitions

The following definitions apply to some of the aspects described withrespect to some embodiments of the invention. These definitions maylikewise be expanded upon herein.

As used herein, the singular terms “a,” “an,” and “the” include pluralreferents unless the context clearly dictates otherwise. Thus, forexample, reference to a layer can include multiple layers unless thecontext clearly dictates otherwise.

As used herein, the term “set” refers to a collection of one or morecomponents. Thus, for example, a set of layers can include a singlelayer or multiple layers. Components of a set also can be referred to asmembers of the set. Components of a set can be the same or different. Insome instances, components of a set can share one or more commoncharacteristics.

As used herein, the term “adjacent” refers to being near or adjoining.Adjacent components can be spaced apart from one another or can be inactual or direct contact with one another. In some instances, adjacentcomponents can be connected to one another or can be formed integrallywith one another.

As used herein, the terms “connect,” “connected,” and “connection” referto an operational coupling or linking. Connected components can bedirectly coupled to one another or can be indirectly coupled to oneanother, such as via another set of components.

As used herein, the terms “substantially” and “substantial” refer to aconsiderable degree or extent. When used in conjunction with an event orcircumstance, the terms can refer to instances in which the event orcircumstance occurs precisely as well as instances in which the event orcircumstance occurs to a close approximation, such as accounting fortypical tolerance levels of the manufacturing operations describedherein.

As used herein, the terms “electrically conductive” and “electricalconductivity” refer to an ability to transport an electric current,while the terms “electrically nonconductive” and “electricalnonconductivity” refer to a lack of ability to transport an electriccurrent. Electrically conductive materials typically correspond to thosematerials that exhibit little or no opposition to flow of an electriccurrent, while electrically nonconductive materials typically correspondto those materials within which an electric current has little or notendency to flow. One measure of electrical conductivity (or electricalnonconductivity) is in terms of Siemens per meter (“S·m⁻¹”). Typically,an electrically conductive material is one having a conductivity greaterthan about 10⁴ S·m⁻¹, such as at least about 10⁵ S·m⁻¹ or at least about10⁶ S·m⁻¹, while an electrically nonconductive material is one having aconductivity less than about 10⁴ such as less than or equal to about 10³S·m⁻¹ or less than or equal to about 10² S−m⁻¹. Electrical conductivity(or electrical nonconductivity) of a material can sometimes vary withtemperature. Unless otherwise specified, electrical conductivity (orelectrical nonconductivity) of a material is defined at roomtemperature.

As used herein with respect to photoluminescence, the term “quantumefficiency” refers to a ratio of the number of output photons to thenumber of input photons.

As used herein, the term “size” refers to a characteristic dimension. Inthe case of a particle that is spherical, a size of the particle canrefer to a diameter of the particle. In the case of a particle that isnon-spherical, a size of the particle can refer to an average of variousorthogonal dimensions of the particle. Thus, for example, a size of aparticle that is a spheroidal can refer to an average of a major axisand a minor axis of the particle. When referring to a set of particlesas having a particular size, it is contemplated that the particles canhave a distribution of sizes around that size. Thus, as used herein, asize of a set of particles can refer to a typical size of a distributionof sizes, such as an average size, a median size, or a peak size.

As used herein, the term “alkane” refers to a saturated hydrocarbonmolecule. For certain applications, an alkane can include from 1 to 100carbon atoms. The term “lower alkane” refers to an alkane that includesfrom 1 to 20 carbon atoms, such as, for example, from 1 to 10 carbonatoms, while the term “upper alkane” refers to an alkane that includesmore than 20 carbon atoms, such as, for example, from 21 to 100 carbonatoms. The term “branched alkane” refers to an alkane that includes oneor more branches, while the term “unbranched alkane” refers to an alkanethat is straight-chained. The term “cycloalkane” refers to an alkanethat includes one or more ring structures. The term “heteroalkane”refers to an alkane that has one or more of its carbon atoms replaced byone or more heteroatoms, such as, for example, N, Si, S, O, and P. Theterm “substituted alkane” refers to an alkane that has one or more ofits hydrogen atoms replaced by one or more substituent groups, such as,for example, halo groups, hydroxy groups, alkoxy groups, carboxy groups,thio groups, alkylthio groups, cyano groups, nitro groups, amino groups,alkylamino groups, dialkylamino groups, silyl groups, and siloxy groups,while the term “un-substituted alkane” refers to an alkane that lackssuch substituent groups. Combinations of the above terms can be used torefer to an alkane having a combination of characteristics. For example,the term “branched lower alkane” can be used to refer to an alkane thatincludes from 1 to 20 carbon atoms and one or more branches. Examples ofalkanes include methane, ethane, propane, cyclopropane, butane,2-methylpropane, cyclobutane, and charged, hetero, or substituted formsthereof.

As used herein, the term “alkyl group” refers to a monovalent form of analkane. For example, an alkyl group can be envisioned as an alkane withone of its hydrogen atoms removed to allow bonding to another group of amolecule. The term “lower alkyl group” refers to a monovalent form of alower alkane, while the term “upper alkyl group” refers to a monovalentform of an upper alkane. The term “branched alkyl group” refers to amonovalent form of a branched alkane, while the term “unbranched alkylgroup” refers to a monovalent form of an unbranched alkane. The term“cycloalkyl group” refers to a monovalent form of a cycloalkane, and theterm “heteroalkyl group” refers to a monovalent form of a heteroalkane.The term “substituted alkyl group” refers to a monovalent form of asubstituted alkane, while the term “un-substituted alkyl group” refersto a monovalent form of an unsubstituted alkane. Examples of alkylgroups include methyl, ethyl, n-propyl, isopropyl, cyclopropyl, butyl,isobutyl, t-butyl, cyclobutyl, and charged, hetero, or substituted formsthereof.

As used herein, the term “arene” refers to an aromatic hydrocarbonmolecule. For certain applications, an arene can include from 5 to 100carbon atoms. The term “lower arene” refers to an arene that includesfrom 5 to 20 carbon atoms, such as, for example, from 5 to 14 carbonatoms, while the term “upper arene” refers to an arene that includesmore than 20 carbon atoms, such as, for example, from 21 to 100 carbonatoms. The term “monocyclic arene” refers to an arene that includes asingle aromatic ring structure, while the term “polycyclic arene” refersto an arene that includes more than one aromatic ring structure, suchas, for example, two or more aromatic ring structures that are bondedvia a carbon-carbon single bond or that are fused together. The term“heteroarene” refers to an arene that has one or more of its carbonatoms replaced by one or more heteroatoms, such as, for example, N, Si,S, O, and P. The term “substituted arene” refers to an arene that hasone or more of its hydrogen atoms replaced by one or more substituentgroups, such as, for example, alkyl groups, alkenyl groups, alkynylgroups, iminyl groups, halo groups, hydroxy groups, alkoxy groups,carboxy groups, thio groups, alkylthio groups, cyano groups, nitrogroups, amino groups, alkylamino groups, dialkylamino groups, silylgroups, and siloxy groups, while the term “un-substituted arene” refersto an arene that lacks such substituent groups. Combinations of theabove terms can be used to refer to an arene having a combination ofcharacteristics. For example, the term “monocyclic lower alkene” can beused to refer to an arene that includes from 5 to 20 carbon atoms and asingle aromatic ring structure. Examples of arenes include benzene,biphenyl, naphthalene, pyridine, pyridazine, pyrimidine, pyrazine,quinoline, isoquinoline, and charged, hetero, or substituted formsthereof.

As used herein, the term “aryl group” refers to a monovalent form of anarene. For example, an aryl group can be envisioned as an arene with oneof its hydrogen atoms removed to allow bonding to another roup of amolecule. The term “lower aryl group” refers to a monovalent form of alower arene, while the term “upper aryl group” refers to a monovalentform of an upper arene. The term “monocyclic aryl group” refers to amonovalent form of a monocyclic arene, while the term “polycyclic arylgroup” refers to a monovalent form of a polycyclic arene. The term“heteroaryl group” refers to a monovalent form of a heteroarene. Theterm “substituted aryl group” refers to a monovalent form of asubstituted arene, while the term “un-substituted arene group” refers toa monovalent form of an unsubstituted arene. Examples of aryl groupsinclude phenyl, biphenylyl, naphthyl, pyridinyl, pyridazinyl,pyrimidinyl, pyrazinyl, quinolyl, isoquinolyl, and charged, hetero, orsubstituted forms thereof.

As used herein, the term “arylene group” refers to a bivalent form of anarene. For example, an arylene group can be envisioned as an arene withtwo of its hydrogen atoms removed to allow bonding to one or moreadditional groups of a molecule. The term “lower arylene group” refersto a bivalent form of a lower arene, while the term “upper arylenegroup” refers to a bivalent form of an upper arene. The term “monocyclicarylene group” refers to a bivalent form of a monocyclic arene, whilethe term “polycyclic arylene group” refers to a bivalent form of apolycyclic arene. The term “heteroarylene group” refers to a bivalentform of a heteroarene. The term “substituted arylene group” refers to abivalent form of a substituted arene, while the term “un-substitutedarylene group” refers to a bivalent form of an unsubstituted arene.Examples of arylene groups include phenylene, biphenylylene,naphthylene, pyridinylene, pyridazinylene, pyrimidinylene, pyrazinylene,quinolylene, isoquinolylene, and charged, hetero, or substituted formsthereof.

Conformal Thin-Film Phosphor Deposition Process

Certain embodiments of the invention relate to a thin-film conformalphosphor deposition process for phosphor-converted white LEDs. Thisprocess facilitates achieving the goal of increasing light scatteringefficiency for white LEDs by providing a thin-film and remote phosphorlayer configuration. According to the improved process as depicted inFIG. 4, a thin-film phosphor layer can be substantially uniformly andconformally deposited on a flat or non-flat and electrically conductive,semiconductive, or nonconductive surface by two operations: (1) forminga phosphor powder layer that is substantially uniformly deposited on thesubstrate surface (operation 42); and (2) forming a polymer binder layerto fill gaps among phosphor particles to form a substantially continuouslayer of thin film (operation 44). The deposition process of forming thethin-film phosphor layer is desirably held in a vacuum chamber. However,it will be appreciated that the deposition process also can take placein a deposition chamber filled with an inert gas, such as nitrogen, orin an atmospheric environment.

In accordance with the process of FIG. 4, a variety of phosphors can beused. Typically, a phosphor is formed from a luminescent material,namely one that emits light in response to an energy excitation.Luminescence can occur based on relaxation from excited electronicstates of atoms or molecules and, in general, can include, for example,chemiluminescence, electroluminescence, photoluminescence,thermoluminescence, triboluminescence, and combinations thereof. Forexample, in the case of photoluminescence, which can includefluorescence and phosphorescence, an excited electronic state can beproduced based on a light excitation, such as absorption of light.Phosphors useful in accordance with the process of FIG. 4 include avariety of inorganic host materials doped by activator ions such as Ce³⁺and Eu²⁺, including garnets (e.g.,(Y_(1-a)Gd_(a))₃(Al_(1-b)Ga_(b))₅O₁₂:Ce³⁺ with a, b≦0.2 or YAG:Ce³⁺),silicates, orthosilicates, sulfides, and nitrides. Garnets andorthosilicates can be used as yellow-emitting phosphors, and nitridescan be used as red-emitting phosphors. However, it will be appreciatedthat various other types of wavelength-conversion materials can be used,including organic dyes. Desirably, phosphors and other types ofwavelength-conversion materials can exhibit photoluminescence with aquantum efficiency that is greater than about 30 percent, such as atleast about 40 percent, at least about 50 percent, at least about 60percent, at least about 70 percent, or at least about 80 percent, andcan be up to about 90 percent or more.

Typically, a phosphor used in accordance with FIG. 4 is provided in apowder form, namely as a set of particles. To enhance color uniformity,the particles desirably have sizes in the range of about 1 nm to about100 μm, such as from about 10 nm to about 30 μm, from about 100 nm toabout 30 μm, from about 500 nm to about 30 μm, or from about 1 μm toabout 30 μm.

Referring to operation 42 of FIG. 4, the phosphor powder can betransported and deposited over the substrate surface by inertia effects,Brownian movement, thermophorcsis, or electrical fields if the phosphorpowder is electrically charged. One approach to form a substantiallyuniformly distributed phosphor powder layer on the substrate surface isto entrain, carry, mobilize, or transport the phosphor powder from aphosphor canister by a set of carrier gases, such as clean, dry air oran inert gas such as nitrogen, and then spray the phosphor powderthrough a showerhead mechanism in a vacuum, inert gas, or atmosphericchamber. For some embodiments, it is desirable that the phosphor powderis ionized with the same positive or negative electrostatic chargeduring the phosphor transport process. When the charged phosphor powderis sprayed and deposited on the substrate surface, the constituentphosphor particles are substantially uniformly distributed to form aphosphor powder layer resulting from self-balancing of electrostaticforces among the phosphor particles. Specifically, electrostaticspraying of the phosphor powder involves:

-   -   1) The phosphor powder is transported by an inert carrier gas        from a phosphor powder canister or other phosphor powder source.        Phosphor powder flow volume can be precisely controlled by a        nozzle device or other flow control mechanism, as depicted in        operation 421 of FIG. 5.    -   2) The phosphor powder is ionized with the same electrostatic        charge, as depicted in operation 422 of FIG. 5. The operation of        ionizing the phosphor powder is desirable to substantially        uniformly deposit the phosphor powder on the substrate surface.        It will be appreciated, however, that this powder ionization        operation is optional, and can be omitted for certain        embodiments.    -   3) As depicted in operation 423 of FIG. 5, if the substrate        surface is formed of an electrically nonconductive polymer        material, the substrate surface is ionized with an opposite        electrostatic charge on the substrate surface. If the substrate        surface is formed of an electrically conductive material, the        substrate surface is grounded, such as by electrically        connecting the substrate surface to a ground potential. The        operation of ionizing or grounding the substrate surface is        desirable to substantially uniformly deposit the phosphor powder        on the substrate surface. It will be appreciated, however, that        this substrate surface ionizing or grounding operation is        optional, and can be omitted for certain embodiments.    -   4) The carrier gas entrains the charged phosphor powder to the        deposition chamber through a showerhead mechanism, thereby        evenly distributing the phosphor powder, as depicted in        operation 424 of FIG. 5. The showerhead mechanism is desirable        to substantially uniformly deposit the phosphor powder on the        substrate surface. Alternatively, or in conjunction, the        substrate surface is rotated in the deposition chamber using a        rotational mechanism so that the phosphor powder can be        substantially uniformly deposited on the substrate surface. It        will be appreciated, however, that these mechanisms are        optional, and can be omitted for certain embodiments.    -   5) The phosphor powder is conformally and substantially        uniformly deposited onto the substrate surface, as depicted in        operation 425 of FIG. 5. In one embodiment, the substrate        surface is a surface of an LED chip or surfaces of multiple LED        chips. In another embodiment, the substrate surface is a surface        of an LED lens or surfaces of multiple LED lenses. In another        embodiment, the substrate surface is a surface of a glass or        quartz substrate. In another embodiment, the substrate surface        is a surface of a flexible transparent film, such as one formed        of polyethylene terephthalate).    -   6) The phosphor powder is discharged with ionizing (or        de-ionizing) gas, as depicted in operation 426 of FIG. 5. The        ionizing gas neutralizes residual electrostatic charges on the        phosphor power. It will be appreciated that this discharging        operation is optional, and can be omitted for certain        embodiments, such as those in which the ionizing operation 422        is omitted.

In the case that the operation 422 of FIG. 5 is implemented, thephosphor powder is ionized with electrostatic charges by one, or acombination, of the following methods:

-   -   Corona charging where electric power is used to generate the        electrostatic charges    -   Triboelectric charging where the electrostatic charges are        generated by friction between the powder and some conduit        surface    -   Induction charging where the powder is charged by induction from        an electrical field

In the case that the operation 423 is implemented for an electricallyconductive substrate, the substrate surface can be grounded to maintainan electric field potential for the deposition of the electrostaticallycharged phosphor powder. Electrostatic charges also can be created onthe phosphor powder or an electrically nonconductive substrate surfaceby Tribo frictional charging. In particular, when two differentmaterials are brought into contact, there can be a transfer of chargefrom one to the other to offset an imbalance of charges. The magnitudeand direction of the charge transfer can depend on a number of factors,including a chemical and electronic structure of both materials. Table 1sets forth certain materials ranging from those with the most positivecharging effect to those with the most negative charging effect whenbrought into contact.

TABLE 1 Materials ordered by Triboelectric Charging Effects MostPositive (+) Air Human Hands, Skin Asbestos Rabbit Fur +++ Glass HumanHair Mica Nylon Wool Lead Cat Fur + Silk Aluminum Paper Cotton SteelWood Lucite Sealing Wax Amber Rubber Balloon Hard Rubber Mylar − NickelCopper Silver UV Resist Brass Synthetic Rubber Gold, Platinum SulfurAcetate, Rayon Polyester Celluloid Polystyrene Orlon, Acrylic CellophaneTape Polyvinylidene chloride (Saran) Polyurethane −−− PolyethylenePolypropylene Polyvinylchloride (Vinyl) Kel-F (PCTFE) Silicon TeflonSilicone Rubber Most Negative (−)

An opposite electrostatic charge can be created on an electricallynonconductive substrate surface with the Tribo frictional chargingmethod. For example, negative charges can be created on thenonconductive substrate surface by one, or a combination, of thefollowing:

-   -   Tribo frictional charging is carried out using a Teflon powder        blown through a nonconductive epoxy or silicone resin surface.        The Teflon powder can carry electrons away from the epoxy or        silicone resin surface to render the surface negatively charged.    -   An epoxy surface is rubbed with a Nylon brush or cloth.

The phosphor deposition process provides a number of advantages,including:

-   -   It can be applied to both a near phosphor configuration and a        remote phosphor configuration for phosphor-converted white LEDs.    -   It can be implemented as a layer-by-layer phosphor deposition        process, and can be readily used to form a multi-color phosphor        thin-film stack.    -   The deposition process can be a dry and clean process, without        any solvents.    -   Controlled quantities of phosphors can be used during        deposition, thereby significantly reducing color variations and        binning issues of white LEDs.    -   It can achieve a substantially uniform coating of phosphors by        introducing electrostatic charges among phosphor particles.    -   It can achieve a high phosphor utilization yield during        deposition.

Polymer Layer Deposition Process

In accordance with the thin-film phosphor deposition process of FIG. 4,the deposited phosphor layer is initially a loosely packed powder layer.Next, a polymer thin film is deposited to fill gaps among phosphorparticles and to form a substantially continuous thin-film layer, asdepicted in operation 44 of FIG. 4. To preserve the substantiallyuniformly distributed phosphor layer structure, it is desirable to use aChemical Vapor Deposition (“CVD”) process to form this polymer layer asa binder material for the phosphor particles. It will be appreciatedthat another suitable deposition process can be used in place of, or inconjunction with, CVD to form the polymer layer. Examples of otherdeposition processes include other vapor deposition processes, such asthermal evaporation, electron-beam evaporation, or physical vapordeposition, as well as spray coating, dip coating, web coating, wetcoating, and spin coating.

FIG. 6A through FIG. 6D depict examples of a family of conformal coatingpolymers that can be used to form a binding matrix for a thin-filmphosphor layer, according to an embodiment of the invention. Inparticular, the family of polymers corresponds to a family ofparylene-based polymers. Generally, parylene-based polymers correspondto a variety of polyxylylene-based polymers, such as poly(p-xylylene)and its derivatives, and include, for example, polymers having a generalrepeating unit of the formula —CZZ′—Ar—CZ″Z′″—, wherein Ar is an arylenegroup (e.g., un-substituted, partially substituted, or fully substitutedarylene group, such as phenylene), and wherein Z, Z′, Z″, and Z′″ can bethe same or different. In specific embodiments, Ar is C₆H_(4-x)X_(x),wherein X is a halogen such as Cl or F, x=0, 1, 2, 3, or 4, and Z, Z′,Z″, and Z′″ are independently selected from H, F, alkyl groups, and arylgroups (e.g., C₆H_(5-x)F_(x) with x=0, 1, 2, 3, 4, or 5). In onespecific embodiment, Parylene N depicted in FIG. 6A includes a repeatingunit of the formula —CH₂—C₆H₄—CH₂—, and is used as a binder material toform a thin-film phosphor layer. In another embodiment, Parylene Cdepicted in FIG. 6B including a repeating unit of the formula—CH₂—C₆H₃Cl—CH₂— is used as a binder material to form a thin-filmphosphor layer. Parylene C can be produced from the same monomer asParylene N, but modified with the substitution of a chlorine atom forone of the aromatic hydrogens. In another embodiment, Parylene Ddepicted in FIG. 6C including a repeating unit of the formula—CH₂—C₆H₂Cl₂—CH₂— is used as a binder material to form a thin-filmphosphor layer. Parylene D can be produced from the same monomer asParylene N, but modified with the substitution of two chlorine atoms fortwo of the aromatic hydrogens. In another embodiment, a partiallyfluorinated parylene-based polymer referred to as Parylene F can beused, as depicted in FIG. 6D. Parylene includes a repeating unit of theformula —CF₂—C₆H₄—CF₂—, and can be formed from various precursors, suchas BrCF₂—C₆H₄—CF₂Br. It will be appreciated that these parylene-basedpolymers are provided by way of example, and a variety of otherconformal coating polymers can be used. Examples of other suitablepolymers include polyimides, fluorocarbon-based polymers (e.g.,poly(tetrafluoroethylene)), poly(p-phenylene vinylene), poly(pyrrole),poly(thiophene), poly(2,4-hexadiyn-1,6-diol), fluorocarbon/organosiliconcopolymers, poly(ethylene glycol), and their derivatives. Thermalevaporation of acrylics also can be used to form a substantiallycontinuous phosphor film.

Various parylene-based polymer films and other types of polymer filmscan be formed via a CVD technique of transport polymerization. Transportpolymerization typically involves generating a vapor phase reactiveintermediate from a precursor molecule at a location remote from asubstrate surface, and then transporting the vapor phase reactiveintermediate to the substrate surface. The substrate surface can be keptbelow a melting temperature of reactive intermediates forpolymerization. For example, Parylene F can be formed from the precursorBrCF₂—C₆H₄—CF₂Br by the removal of the bromine atoms to form thereactive intermediate *CF₂—C₆H₄—CF₂*, wherein * denotes a free radical.This reactive intermediate can be formed at a location remote from adeposition chamber, and can be transported into the deposition chamberand condensed over the substrate surface, where polymerization takesplace.

More generally, parylene-based polymer films can be formed from avariety of precursors, such as those having the formula(CZZ′Y)_(m)—Ar—(CZ″Z′″Y′)_(n), wherein Ar is an arylene group (e.g.,un-substituted, partially substituted, or fully substituted arylenegroup, such as phenylene), Z, Z′, Z″, and Z′″ can be the same ordifferent, Y and Y′ can be the same or different and are removable togenerate free radicals, m and n are each equal to zero or a positiveinteger, and a sum of m and n is less than or equal to a total number ofsp²-hybridized carbons on Ar available for substitution. In specificembodiments, Ar is C₆H_(4-x)X_(x), wherein X is a halogen such as Cl orF, x=0, 1, 2, 3, or 4, and Z, Z′, Z″, and Z′″ are independently selectedfrom H, F, alkyl groups, and aryl groups (e.g., C₆H_(5-x)F_(x) with x=0,1, 2, 3, 4, or 5). Other suitable precursors include dimers having theformula {(CZZ′)—Ar—(CZ″Z′″)}₂, wherein Ar is an arylene group (e.g.,un-substituted, partially substituted, or fully substituted arylenegroup, such as phenylene), and Z, Z′, Z″, and Z′″ can be the same ordifferent. In specific embodiments, Ar is C₆H_(4-x)X_(x), wherein X is ahalogen such as Cl or F, x=0, 1, 2, 3, or 4, and Z, Z′, Z″, and Z′″ areindependently selected from H, F, alkyl groups, and aryl groups (e.g.,C₆H_(5-x)F_(x) with x=0, 1, 2, 3, 4, or 5).

One aspect of a parylene-based polymer film, or another type of polymerfilm, prepared by the CVD method is that it is a conformal coating withsuperior crevice penetration capability, thereby substantially fillinggaps and voids within a phosphor powder layer. In some instances,Parylene F can achieve the best result for gap-filling, while Parylene Ncan achieve the second best result for gap-filling among the family ofparylene-based polymers. Another aspect of a parylene-based polymer isthat it has superior optical transparency in the visible light spectrum,rendering it a suitable filler material among a photoluminescentphosphor powder. Another aspect of a parylene-based polymer is that itsrefractive index can be adjusted based on chemical composition. In oneembodiment, a multi-layer of parylene-based polymer films can be formedas a composite thin-film phosphor stack. This multi-layer structure canbe formed by depositing a Parylene N film, with a refractive index ofabout 1.66, as a binder material among a phosphor powder, and thendepositing a Parylene F film, with a refractive index of about 1.4,thereby enhancing light extraction due to index matching of the ParyleneF film to ambient environment (e.g., air). It will be appreciated that,in general, this multi-layer structure can be formed by depositing afirst polymer film, with a first refractive index, as a binder materialamong a first phosphor powder layer to form a first phosphor layeradjacent to the substrate surface, depositing a second polymer film,with a second refractive index, as a binder material among a secondphosphor powder layer to form a second phosphor layer adjacent to thefirst phosphor layer, and so on, where the first refractive index isgreater than or equal to the second refractive index.

Using the CVD method, a parylene-based polymer, or another type ofpolymer, can be formed as a substantially continuous film having athickness in the range of a few tens of angstroms to about 100 μm, suchas from about 1 nm to about 100 μm, from about 10 nm to about 100 μm,from about 100 nm to about 100 μm, from about 1 μm to about 100 μm, fromabout 1 μm to about 75 μm, from about 1 μm to about 30 μm, or from about1 μm to about 10 μm. In some instances, the thickness of the film canexhibit a standard deviation of less than about 20 percent with respectto an average thickness, such as less than about 10 percent or less thanabout 5 percent. A thickness of the initially deposited phosphor powderlayer can be in the range of about 1 nm to about 60 μm, such as fromabout 10 nm to about 60 μm, from about 100 nm to about 40 μm, or fromabout 100 nm to about 20 μm. In some instances, the thickness of thephosphor powder layer can exhibit a standard deviation of less thanabout 20 percent with respect to an average thickness, such as less thanabout 10 percent or less than about 5 percent. A distribution of thephosphor powder within the resulting film can be substantially uniformacross an extent of the film, such that a weight density (e.g., mass orweight of phosphor particles per unit volume) or a number density (e.g.,number of phosphor particles per unit volume) can exhibit a standarddeviation of less than about 20 percent with respect to an averagedensity, such as less than about 10 percent or less than about 5percent.

An embodiment of a thin-film phosphor layer prepared by the CVD methodis depicted in FIG. 7A. In FIG. 7A, a single-color phosphor powder layer71, such as a YAG:Ce³⁺-based yellow phosphor, is initially deposited ona substrate surface 72. The substrate surface 72 can be an electricallynonconductive surface, such as a surface of a flexible plasticsubstrate. A parylene-based polymer layer 73 is deposited, and anotherparylene-based polymer layer 74 is next deposited. The parylene-basedpolymer layer 73 serves as a binder or a matrix that at least partiallypenetrates or surrounds the phosphor powder layer 71, such that phosphorparticles of the phosphor powder layer 71 are dispersed within theparylene-based polymer layer 73. It will be appreciated that theparylene-based polymer layers 73 and 74 can be formed from the samematerial or different materials. In some instances, a refractive indexof the parylene-based polymer layer 73 is greater than a refractiveindex of the parylene-based polymer layer 74.

In accordance with a layer-by-layer deposition of phosphor powders, theCVD method can be used to form a substantially uniformly distributedmulti-color phosphor stack. In an embodiment depicted in FIG. 7B, amulti-color phosphor thin-film stack 75 is formed by sequentialdeposition of a blue phosphor powder, a parylene-based polymer as abinder material for the blue phosphor powder, a green phosphor powder, aparylene-based polymer as a binder material for the green phosphorpowder, a red phosphor power, and a parylene-based polymer as a bindermaterial for the red phosphor powder. A resulting phosphor-convertedwhite LED can emit three down-converted secondary lights of respectivecolors by the phosphors. Thus, a Color Rendering Index (“CRI”) of thephosphor-converted white LED can be readily tuned, for example, whenused in an indoor general illumination application with a warmer whitelight and an improved color uniformity. Another application of thephosphor-converted white LED incorporating the multi-color thin-filmphosphor stack 75 is for backlighting of Liquid Crystal Displays(“LCDs”).

It will be appreciated that the phosphor powder deposition in operation42 and the polymer deposition in operation 44 need not take placesequentially. Alternatively, these operations can take placesubstantially simultaneously to form a conformal phosphor thin-filmlayer.

Thin-Film Phosphor Deposition System

FIG. 8 depicts a thin-film phosphor deposition system 80, according toan embodiment of the invention. The thin-film phosphor deposition system80 can be used to form a conformally coated thin-film phosphor layer bydepositing a substantially uniformly distributed phosphor powder layerand then depositing a conformally coated CVD polymer to form asubstantially continuous phosphor thin film. The thin-film phosphordeposition system 80 includes: (1) a deposition subsystem or unit 81;(2) a phosphor powder delivery subsystem or unit 82 connected to thedeposition subsystem 81; and (3) a parylene-based polymer precursordelivery subsystem or unit 83 connected to the deposition subsystem 81.

The deposition subsystem 81 includes: (1) a deposition chamber 81 a,which defines an enclosure within which a substrate is disposed and, forexample, is a vacuum chamber with an associated vacuum pump to maintainvacuum conditions, is filled with an inert gas, or is an atmosphericchamber; (2) a substrate holder 81 b within the chamber 81 a and, forexample, is capable of rotating the substrate during the formation ofthe phosphor film; (3) a showerhead mechanism 81 c; and (4) a phosphorpowder ionizer 81 d, which introduces electrostatic charges amongphosphor particles during phosphor powder deposition.

The phosphor powder delivery subsystem 82 includes: (1) a phosphorpowder canister 82 a or other phosphor powder source; (2) a phosphorflow controller 82 b, which regulates a specified quantity of phosphorsfor each deposition of phosphors; (3) an ionizer 82 c, which introduceselectrostatic charges among phosphor particles during phosphor powderdeposition; and (4) a set of valves 82 d.

The parylene-based polymer precursor delivery subsystem 83 includes: (1)a precursor canister 83 a or other precursor source; (2) a precursorflow controller 83 b, which regulates a specified quantity of precursorsfor each polymer deposition; (3) a gas reactor 83 c, which induces thegeneration of vapor phase reactive intermediates from precursors; and(4) a set of valves 83 d.

In order to deposit parylene-based films, solid or liquid precursors areheated in the stainless canister 83 a to a consistent temperature togenerate vapor phase precursors. The vapor phase precursors are fed intothe gas reactor 83 e, which is regulated by the flow controller 83 b, asdepicted in FIG. 8. The gas reactor 83 c splits the precursors intoreactive intermediates that bear two unpaired electrons, or diradicals.The diradicals are transported to the deposition chamber 81 a shortlyafter the gas reactor 83 c activates the precursors. Because thediradicals can be very reactive, the diradicals can quickly polymerizeto form a polymer thin film once the diradicals collide with each otheron a substrate surface held inside the deposition chamber 81 a.

The thin-film phosphor deposition system 80 also includes a controlsubsystem or unit, which includes a processor 84 and an associatedmemory 85 that are connected to other components of the system 80 andserve to direct operation of those components. The memory 85 can includea computer-readable storage medium having computer code stored thereonfor performing various computer-implemented operations. The media andcomputer code can be those specially designed and constructed for thepurposes of embodiments of the invention, or they may be of the kindwell known and available to those having skill in the computer softwarearts. Examples of computer-readable media include, but are not limitedto: magnetic storage media such as hard disks, floppy disks, andmagnetic tape; optical storage media such as Compact Disc/Digital VideoDiscs (“CD/DVDs”), Compact Disc-Read Only Memories (“CD-ROMs”), andholographic devices; magneto-optical storage media such as flopticaldisks; and hardware devices that are specially configured to store andexecute program code, such as Application-Specific Integrated Circuits(“ASICs”), Programmable Logic Devices (“PLDs”), and ROM and RAM devices.Examples of computer code include, but are not limited to, machine code,such as produced by a compiler, and files containing higher-level codethat are executed by a computer using an interpreter. For example, anembodiment of the invention may be implemented using Java, C++, or otherobject-oriented programming language and development tools. Additionalexamples of computer code include, but are not limited to, encryptedcode and compressed code. Other embodiments of the invention can beimplemented using hardwired circuitry in place of, or in conjunctionwith, computer code.

Advantages of Thin-Film Phosphor Deposition Method

Table 2 sets forth certain advantages of the conformal thin-filmphosphor deposition method of some embodiments of the invention,relative to other phosphor coating methods.

TABLE 2 Comparison of Phosphor Coating Methods Conformal Thin- SlurryMethod EPD Ceramic Plate Film Phosphor Spatial proximate proximateproximate remote conformal Phosphor phosphor-in-cup conformal phosphorphosphor Distribution phosphor Scattering ~50% ~40% Slightly better ≧about 90% Efficiency than EPD (e.g., ≧ about 92% or ≧ about 95% and upto about 99% or more) Homogeneity Poor Good Good Good Color Poor GoodGood Good Consistency Color Possible for Single Possible for Possiblefor layer- Rendering multi-color phosphor multi-layer by-layer phosphorphosphor ceramic plate deposition Temperature Medium Poor Good BestStability Cost Die-level Batch process Die-level Batch process processprocess

In conjunction with the advantages set forth above, a light emittingdevice formed in accordance with the conformal thin-film phosphordeposition method can emit white light of greater uniformity. Inparticular, a CCT variation of a white light LED can be no greater thanabout 1,000 K over a 140° (±70° from a center light-emitting axis) rangeof light emission angles, such as no greater than about 800 K, nogreater than about 500 K, or no greater than about 300 K, and down toabout 200 K or less.

While the invention has been described with reference to the specificembodiments thereof, it should be understood by those skilled in the artthat various changes may be made and equivalents may be substitutedwithout departing from the true spirit and scope of the invention asdefined by the appended claims. In addition, many modifications may bemade to adapt a particular situation, material, composition of matter,method, or process to the objective, spirit and scope of the invention.All such modifications are intended to be within the scope of the claimsappended hereto. In particular, while the methods disclosed herein havebeen described with reference to particular operations performed in aparticular order, it will be understood that these operations may becombined, sub-divided, or re-ordered to form an equivalent methodwithout departing from the teachings of the invention. Accordingly,unless specifically indicated herein, the order and grouping of theoperations are not limitations of the invention.

1-16. (canceled)
 17. A system to form a thin-film phosphor layer on asubstrate, the system comprising: a deposition subsystem defining anenclosure to accommodate the substrate; a phosphor powder deliverysubsystem configured to deliver, using a carrier gas, a phosphor powderfrom a source of the phosphor powder to the deposition subsystem; apolymer precursor delivery subsystem configured to deliver polymerprecursors in a vapor phase to the deposition subsystem; and a controlsubsystem connected to the deposition subsystem, the phosphor powderdelivery subsystem, and the polymer precursor delivery subsystem,wherein the control subsystem is configured to control the phosphorpowder delivery subsystem to deliver the phosphor powder to thedeposition subsystem for a first time interval to form a phosphor powderlayer adjacent to the substrate, and the control subsystem is configuredto control the polymer precursor delivery subsystem to deliver thepolymer precursors to the deposition subsystem for a second timeinterval to form a polymer layer adjacent to the phosphor powder layer.18. The system of claim 17, wherein the deposition subsystem includes achamber defining the enclosure, a substrate holder configured to supportthe substrate within the chamber, and a showerhead mechanism configuredto deposit the phosphor powder over the substrate.
 19. The system ofclaim 18, wherein the substrate holder is configured to rotate thesubstrate.
 20. The system of claim 18, wherein the deposition subsystemfurther includes an ionizer.
 21. The system of claim 17, wherein thephosphor powder delivery subsystem includes an ionizer.
 22. The systemof claim 17, wherein the polymer precursor delivery subsystem includes agas reactor configured to generate reactive intermediates in a vaporphase from the polymer precursors.
 23. The system of claim 22, whereinthe gas reactor is configured to generate free radicals from the polymerprecursors, and the polymer precursors have the formula:(CZZ′Y)_(m)—Ar—(CZ″Z′″Y′)_(n), wherein Ar is selected from (1) anun-substituted phenylene group, (2) a chlorine-substituted phenylenegroup of the formula: C₆H_(4-x)Cl_(x), with x being an integer in therange of 1 to 4, and (3) a fluorine-substituted phenylene group of theformula: C₆H_(4-x)F_(x′), with x′ being an integer in the range of 1 to4, Z, Z′, Z″, and Z′″ are independently selected from H, F, alkylgroups, and aryl groups, Y and Y′ being removable to generate the freeradicals, m and n are each equal to zero or a positive integer, and asum of m and n is less than or equal to a total number of sp²-hybridizedcarbons on Ar available for substitution.
 24. The system of claim 22,wherein the gas reactor is configured to generate free radicals from thepolymer precursors, and the polymer precursors include dimers having theformula: {(CZZ′)—Ar—(CZ″Z′″)}₂, wherein Ar is selected from (1) anun-substituted phenylene group, (2) a chlorine-substituted phenylenegroup of the formula: C₆H_(4-x)Cl_(x), with x being an integer in therange of 1 to 4, and (3) a fluorine-substituted phenylene group of theformula: C₆H_(4-x′)F_(x′), with x′ being an integer in the range of 1 to4, and Z, Z′, Z″, and Z′″ are independently selected from H, F, alkylgroups, and aryl groups. 25-26. (canceled)